Measuring voltage is easy. Just turn the dial to a voltage range higher than you will be working with - usually the 20 volt DC range for RV work. The meter should handle reverse polarity by showing a minus sign on the voltage so no worries about which lead you touch to positive or ground.
Measuring current is more complicated. Best to have a knowledgeable person watch you do your first measurements. Choose a high current range to begin with - too low and the overcurrent may destroy that range. If you connect the meter as you would a voltmeter, you will certainly blow the current range on a low cost meter because current meters offer very low resistance to the current and very high current will flow through until something burns out. The thing is, in order to measure current the current has to flow through the meter. So you must unhook or cut the wire and use the leads to make the current flow through the load AND meter.
My low cost multimeters came with poorly made probes; the lead always breaks off the probe handle. I solder them back together with some difficulty. I finally realized that it is much easier to solder an alligator clip on instead of a probe - and the clip is usually more convenient in use.
2004 E350 Adventurer (Canadian) 20 footer - Alberta, Canada
No TV + 100W solar = no generator needed